which switch?

There was a millionaire with 10 bedrooms on the 2nd floor of his house.
Each bedroom has one closet and one bathroom. Only one(1) closet has a light bulb. To be able to turn on the light bulb, he needs to go downstairs to turn on the switch. There were three (3) switches,only one (1) switch will turn on the light bulb. The other two (2) are useless. You can only go upstairs once. How do you figure out which switch works?